1. Field of the Invention
The present invention relates to an exhaust gas purification device comprising a NOx trap catalyst for an internal combustion engine. More particularly, the invention relates to improving the NOx reduction performance of the device.
2. Description of the Related Art
JP-A 2001-59440 discloses an exhaust gas purification device comprising a NOx trap catalyst. The catalyst has functions of trapping and storing nitrogen oxides (NOx) contained in exhaust gas, when an air-fuel ratio of exhaust gas is leaner than stoichiometric air-fuel ratio. The catalyst has other functions of releasing and reducing NOx, when the air-fuel ratio of exhaust gas is richer than stoichiometric air-fuel ratio. In this prior art, in order to make the air-fuel ratio of exhaust gas rich, after performing a main fuel injection, an additional fuel injection is performed.
JP-A 2000-154713 discloses that the performance related to NOx reduction improves by supplying hydrogen (H2) to the NOx trap catalyst.
In diesel engine, it is possible to supply H2 to the NOx trap catalyst by performing an additional fuel injection immediately after a main fuel injection. But, by performing such additional fuel injection, there are cases where the engine output torque varies.
The present invention provides a solution to the above-mentioned torque variation.
According to present invention, there is provided an exhaust gas purification device for diesel engine. The engine has an injector injecting fuel directly into combustion chamber of the engine. The exhaust gas purification device comprises a NOx trap catalyst and a controller. The NOx trap catalyst traps nitrogen oxides (NOx) contained in exhaust gas when an air-fuel ratio of exhaust gas is leaner than stoichiometric air-fuel ratio, and reduces NOx stored in the catalyst when the air-fuel ratio of exhaust gas is richer than stoichiometric air-fuel ratio. The controller is programmed to make a judgment as to whether the catalyst should be regenerated; calculate instruction values regarding the main fuel injection in accordance with engine operating conditions and the judgment; calculate instruction values regarding additional fuel injection, when the judgment shows that the catalyst should be regenerated; and control the injector according to the instruction values about the main and the additional fuel injection, in order to regenerate the catalyst.